The present disclosure relates to methods and apparatus for improving reliability of a cover substrate against impact fractures, for example, for substrates including glass substrates, crystalline substrates, single crystal substrates, glass ceramic substrates, etc.
Many consumer and commercial products employ a sheet of high-quality cover glass to protect critical devices within the product, provide a user interface for input and/or display, and/or many other functions. For example, mobile devices, such as smart phones, mp3 players, computer tablets, etc., often employ one or more sheets of high strength glass on the product to both protect the product and achieve the aforementioned user interface. In 2013, more than 1 billion smartphones were shipped worldwide, which represented a 40% increase from the previous year. Some have predicted that 1.7 billion smartphones will be shipped in 2017.
In the above-noted applications, as well as others, the glass is preferably durable (e.g., scratch resistant and fracture resistant), transparent, and/or antireflective. Indeed, in a smart phone and/or tablet application, the cover glass is often the primary interface for user input and display, which means that the cover glass would preferably exhibit high durability and high optical performance characteristics. Among the evidence that the cover glass on a product may manifest exposure to harsh operating conditions, fractures (e.g., cracks) and scratches are probably the most common. Such evidence suggests that sharp contact, single-event damage is the primary source of visible cracks (and/or scratches) on cover glass in mobile products. Since 2007, some have estimated that damaged smartphones have cost consumers in America about 5.9 billion dollars, where 76% of such damage was caused by dropping the smartphone to the ground.
Once a significant crack or scratch mars the cover glass of a user input/display element, the appearance of the product is degraded and the resultant increase in light scattering may cause significant reduction in the performance of the display. Significant cracks and/or scratches can also affect the accuracy and reliability of touch sensitive displays. As a single severe crack and/or scratch, and/or a number of moderate cracks and/or scratches, are both unsightly and can significantly affect product performance, they are often the leading complaint of customers, especially for mobile devices such as smart phones and/or tablets.
In order to reduce the likelihood of cracking and/or scratching the cover glass of a product, it has been proposed to increase the hardness of the cover glass to about 15 GPa or higher. One approach to increasing the hardness of a given glass substrate is to apply a film coating or layer to the glass substrate to produce a composite structure that exhibits a higher hardness as compared to the bare glass substrate. For example, a diamond-like carbon coating may be applied to a glass substrate to improve hardness characteristics of the composite structure. Indeed, diamond exhibits a hardness of 100 GPa; however, such material is used sparingly due to high material costs. Further, although a coating on a glass substrate may improve the hardness of the structure, and thereby improve the resistance to cracks and/or scratches, it has been discovered that the coating may degrade other characteristics of the substrate, such as the flexural strength of the glass substrate and/or the strain to failure of the glass substrate. The reduction in the strength and/or strain to failure of the glass substrate may manifest in a higher susceptibility to cracks, particularly deep cracks.
Accordingly, there are needs in the art for new methods and apparatus for achieving high hardness coatings on substrates, such as glass substrates, crystalline substrates, single crystal substrates, glass ceramic substrates, etc., and to avoid failure of the substrates in the even of an impact.